Container sealants are needed to preserve the quality of canned and bottled goods. At present, plastisols based on PVC are widely used because they offer good technical solutions to a number of problems faced by canners and bottlers. Plastisols made with PVC are flexible, exhibit good adhesion to metal, can be formulated as a low viscosity film or coating, do not add a taste or smell to a packaged food or beverage, and are relatively inexpensive to produce. On the other hand, the raw materials do contain chlorine, which has become the subject of environmental concern.
Chlorine, which is essential to life and is found in common table salt, is a highly reactive element which exerts a profound effect on molecules that contain it. Many chlorinated organic compounds are highly reactive, and many, including the vinyl chloride monomer used to make the PVC polymer, are carcinogens. Further, improper incineration of materials that contain PVC can produce hydrogen chloride (a corrosive acid) and dioxins (which are poisonous). Since many communities world-wide are incinerating municipal solid waste, there is a demand to reduce the use of PVC rather than depend upon the proper incineration of waste every time.
It can be seen, therefore, that there is a need for a non-PVC plastisol, particularly where environmental concerns exist.
Plastisols are essentially blends of finely divided thermoplastic polymers suspended in a liquid medium, or plasticizer. Plastisols are particularly useful as an aid to handling polymers which would otherwise exist in a highly viscous or semi-solid state. U.S. Pat. No. 3,925,280, issued to Lundberg et al. Dec. 9, 1975 discloses that broad classes of thermoplastic materials, including graft or block copolymers as well as various ionomers, can be powdered and then blended with plasticizers to produce a suspension which is useful for making thin layers or coatings. The suspension is simply applied to a surface of an article and heated to a softening point of the polymer. A liquid-solid transition occurs upon heating, producing a plasticized semi-rigid or elastomeric product of high strength.
In U.S. Pat. No. 4,014,847, issued to Lundberg et al. Mar. 29, 1977, it is further disclosed that certain polar plasticizers such as glycerol are compatible with various ionomers including sulfonated polystyrenes. No particular end use for the resulting suspensions is disclosed.
U.S. Pat. No. 4,425,455 issued to Turner et al. Jan. 10, 1984 discloses that sulfonated thermoplastic polymers, including copolymers of sodium styrene sulfonate with styrene, vinyl toluene or t-butyl styrene, can be blended with a polar cosolvent (plasticizer) for use in a drilling mud.
A method of making styrene and sodium styrene sulfonate (Sty/NaSS) copolymers is disclosed in J. H. Kim et al., "Preparation of Highly Sulfonated Polystyrene Model Colloids" J. Polymer Sci: Part A: Polymer Chemistry, Vol. 27, 3187-3199 (1989). End uses of the resulting materials are not discussed.
Copending application U.S. Ser. No. 208,764 filed Mar. 8, 1994, and issued as U.S. Pat. No. 5,387,633 on Feb. 7, 1995. "Styrene/Sodium Styrene Sulfonate Copolymers for Plastisol Sealant" disclosed the use of the base copolymers of the present invention for making plastisols.
The present inventor has found that incorporation of small amounts of glycidyl methacrylate (GMA) improves the general mechanical properties of the fused plastisol materials. The mechanism of such improvement likely involves crosslinking chemistry at the process of plastisol fusion, which results in improved integrity of the film.